In an ion implantation apparatus having a conventional acceleration system, operational parameters regarding the acceleration of the ions in the beam can be easily obtained by analysis. For example, in an acceleration method which utilizes an electrostatic field, typical in most ion implantation apparatuses, the required voltage (V) of a power supply which is used to create the electrostatic field is simply obtained by the following equation (1) using the ionic valence value (n) of the desired ions and the desired energy (E) of the ions, typically measured in kilo-electron volts (keV). EQU V=E/n (1)
When the electric field is applied in multiple stages, the sum of all of the fields can be made to be equal to the value V.
However, in an ion implantation apparatus utilizing a radio frequency (RF) linear accelerator (linac), comprised of resonator modules each having an accelerating electrode, both the amplitude (in kilovolts (kV)) and the frequency (in Hertz (Hz)) of the accelerating electrode output signal must be determined as operating parameters of the resonator module. Moreover, when a multiple-stage RF linac is utilized, the phase difference (.PHI.) (in degrees(.degree. )) of each accelerating electrode output signal is included within the required operational parameters.
When a multiple-stage RF linac is used, the amplitude, frequency and phase difference of the accelerating electrode output signals cannot be analytically determined using the incoming energy of the ions into the RF linac and the post-acceleration desired energy of the ions. This is because there are indefinite sets of solutions corresponding to the combination of required parameters.
In addition, when magnets (such as a quadrupole magnet or an electromagnet) are used for controlling the lateral spread of an ion beam during or after acceleration, or when electrostatic lenses (such as electrostatic quadrupole electrodes) are used to provide a convergence/divergence effect on the beam, their operation parameters (e.g., electrical current or voltage) must be also determined. However, such magnetic or electrostatic operational parameters cannot be determined until the RF linac parameters are determined, because the optimum values for these factors are altered depending on the energy of the ions passing therethrough. In addition, the strength of the electric field of the RF linac affects the convergence/divergence of the magnet or electrostatic lens. Furthermore, even after the RF linac parameters (amplitude, frequency, and phase) are determined, the magnetic or electrostatic operational parameters cannot be analytically determined but are instead calculated step by step.
As previously discussed, in an ion implantation apparatus in which ions are accelerated using an electrostatic voltage, acceleration parameters can be easily determined by analysis. Hence, if data such as an acceleration condition (the ionic valence value of ions) and a desired energy is entered by an operator or provided by a higher level computer, the necessary acceleration parameter (e.g., electrical current or voltage) can be calculated by a control device of the ion implantation apparatus and automatically determined by analytical solution of equations. FIG. 5 shows such a process for determining an electrostatic acceleration parameter.
However, in the case of an implantation apparatus including an RF linac, the RF linac operational parameters (amplitude, frequency and phase) and the parameters of a convergence/divergence lens which controls the convergence/divergence of an ion beam cannot be analytically obtained. As shown in FIG. 6, a typical process for determining the linac operational parameters involves first selecting combinations of parameter values that have previously been found to optimize operation of the RF linac for a particular desired target energy level. The selection is based on acceleration conditions and a desired final energy value. If the selected combination of parameter values results in achievement of the target energy value, the selected combination of parameters is used without changes.
If however, as is likely, the target energy value is not achieved using the selected combination of parameter values, the combination of parameter values that comes closest to achieving the target energy value is chosen to actually accelerate an ion beam. Then, by gradually changing the control parameters, a combination of parameters is found for obtaining a beam with the target energy. Through successive iterations, using trial-and-error operations that are necessary because changing one parameter affects the others, the parameters are adjusted gradually until an optimum combination of parameter values are found.
However, the process as shown in FIG. 6 requires a very large amount of time and effort to arrive at the optimum combination of operational parameters. In addition, one cannot be sure that the obtained combination of parameters is the optimum combination. Moreover, the adjustment must be performed by an operator and hence, an automatic start-up and operation cannot be achieved for an ion beam with a new set of operating conditions.
It is therefore a purpose of the present invention to provide quick and easy automatic calculation of RF linac operational parameters for an ion implantation apparatus. Another purpose of the present invention is to enable the generation of an ion beam having a desired energy level in a short period of time. Yet another purpose of the present invention is to enable operating parameters for a convergence/divergence lens in an ion implanter to be established with ease and in a short period of time.